Lithographic apparatus, spectral purity filter and device manufacturing method

ABSTRACT

A lithographic apparatus for patterning a beam of radiation and projecting it onto a substrate, comprising at least two spectral purity filters configured to reduce the intensity of radiation in the beam of radiation in at least one undesirable range of radiation wavelength, wherein the two spectral purity filters are provided with different radiation filtering structures from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application61/449,381, which was filed on 4 Mar. 2011, and which is incorporatedherein in its entirety by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a lithographic apparatus, a spectralpurity filter and a device manufacturing method.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (IC's). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern be formed on an individual layer of the IC.This pattern can be transferred onto a target portion (e.g., includingpart of, one, or several dies) on a substrate (e.g., a silicon wafer).Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

Lithography is widely recognized as one of the key steps in themanufacture of ICs and other devices and/or structures. However, as thedimensions of features made using lithography become smaller,lithography is becoming a more critical factor for enabling miniature ICor other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given bythe Rayleigh criterion for resolution as shown in equation (1):

$\begin{matrix}{{CD} = {k_{1}*\frac{\lambda}{N\; A_{PS}}}} & (1)\end{matrix}$where λ is the wavelength of the radiation used, NA_(PS) is thenumerical aperture of the projection system used to print the pattern,k₁ is a process dependent adjustment factor, also called the Rayleighconstant, and CD is the feature size (or critical dimension) of theprinted feature. It follows from equation (1) that, reduction of theminimum printable size of features can be obtained in three ways: byshortening the exposure wavelength λ, by increasing the numericalaperture NA_(PS) or by decreasing the value of k₁.

In order to shorten the exposure wavelength and, thus, reduce theminimum printable size, it has been proposed to use an extremeultraviolet (EUV) radiation source. EUV radiation sources are configuredto output a radiation wavelength of about 13 nm. Thus, EUV radiationsources may constitute a significant step toward achieving smallfeatures printing. Such radiation is termed extreme ultraviolet or softx-ray, and possible sources include, for example, laser-produced plasmasources, discharge plasma sources, or synchrotron radiation fromelectron storage rings. Along with useful EUV in-band radiation, EUVradiation sources may produce almost equal (and sometimes more)undesirable out-of-band infrared (“IR”) and deep ultraviolet (“DUV”)radiation.

Spectral purity filters have been developed to filter the non-EUVradiation out of the beam of radiation to be used for exposure.

EP 1 717 609 discloses a lithographic apparatus including multi-layermirrors that may each be provided with a respective spectral purityenhancement layer.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus in accordance with an embodimentof the invention;

FIG. 2 depicts a lithographic apparatus in accordance with an embodimentof the invention;

FIG. 3 depicts a radiation source and a normal incidence collector inaccordance with an embodiment of the invention;

FIG. 4 depicts a radiation source and a Schwarzschild type normalincidence collector in accordance with an embodiment of the invention;

FIG. 5 depicts embodiments of a spectral purity filter that may be usedin the lithographic apparatus of FIG. 1;

FIG. 6 depicts an arrangement of a radiation beam conditioning systemusing a spectral purity filter according to the present invention;

FIG. 7 depicts a variant of the arrangement depicted in FIG. 6;

FIGS. 8a to 8g depict a method of forming a spectral purity filer;

FIG. 9 depicts a system for projecting an interference pattern ofradiation onto a target for use in the method depicted in FIGS. 8a to 8;

FIGS. 10 and 11 schematically depict an arrangement of a spectral purityfilter that may be used in the present invention;

FIG. 12 depicts an arrangement of a spectral purity filter that may beused in the present invention;

FIG. 13 depicts an arrangement of a spectral purity filter that may beused in the present invention; and

FIG. 14 depicts an arrangement of a spectral purity filter that may beused in the present invention.

Features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout, in the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts an embodiment of a lithographic apparatus,that can be or include an embodiment of the invention. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., EUV radiation); a support structure orpatterning device support (e.g., a mask table) MT constructed to supporta patterning device (e.g., a mask or a reticle) MA and connected to afirst positioner PM configured to accurately position the patterningdevice; a substrate table (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate; and aprojection system (e.g., a reflective projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam that is reflected by the mirrormatrix.

The term “projection system” may encompass any type of projectionsystem, including refractive, reflective, catadioptric, magnetic,electromagnetic and electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, or forother factors such as the use of an immersion liquid or the use of avacuum. It may be desired to use a vacuum for EUV or electron beamradiation since other gases may absorb too much radiation or electrons.A vacuum environment may therefore be provided to the whole beam pathwith the aid of a vacuum wall and vacuum pumps.

As here depicted, the apparatus is of a reflective type (e.g., employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g., employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source SO may be part of a radiation system 3(i.e., radiation generating unit 3). The radiation system 3 and thelithographic apparatus may be separate entities. In such cases, theradiation system 3 is not considered to form part of the lithographicapparatus and the radiation beam is passed from the source SO ofradiation system 3 to the illuminator IL with the aid of a beam deliverysystem including, for example, suitable directing mirrors and/or a beamexpander. In other cases, the source may be an integral part of thelithographic apparatus.

The source SO of the radiation system 3 may be configured in variousways. For example, the source SO may be a laser produced plasma source(LPP source), for example a Tin LPP source (such LPP sources are knownper se) or a discharge-produced plasma source (DPP source). The sourceSO may also be a different type of radiation source.

The illuminator IL may include an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator and a condenser.The illuminator may be used to condition the radiation beam, to have adesired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF2 (e.g., an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor IF1 can be used to accurately position the patterningdevice (e.g., mask) MA with respect to the path of the radiation beam B.Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure (e.g., mask table) MT and thesubstrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

2. In scan mode, the support structure (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the support structure (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS.

3. In another mode, the support structure (e.g., mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically shows a further embodiment of an EUV lithographicapparatus, having a principle of operation that is similar to theoperation of the apparatus shown in the embodiment of FIG. 1. In theembodiment of FIG. 2, the apparatus includes a source-collector-moduleor radiation unit 3 (also referred to herein as a radiation system), anillumination system IL and a projection system PS. According to anembodiment, radiation unit 3 is provided with a radiation source SO,preferably a laser produced plasma (“LPP”) source. In the presentembodiment, the radiation emitted by radiation source SO may be passedfrom the source chamber 7 into a chamber 8 via, a gas barrier or “foiltrap” 9. In FIG. 2, the chamber 8 includes a radiation collector 10.

FIG. 2 depicts the application of a grazing incidence collector 10.However, the collector may be a normal incidence collector, particularlyin the case the source is a LPP source. In yet another embodiment, thecollector may a Schwarzschild collector (see FIG. 4), and the source maybe a DPP source.

The radiation may be focused in a virtual source point 12 (i.e., anintermediate focus IF) from an aperture in the chamber 8. From chamber8, the radiation beam 16 is reflected in illumination system 11 vianormal incidence reflectors 13,14 onto a patterning device (e.g.,reticle or mask) positioned on support structure or patterning devicesupport (e.g., reticle or mask table) MT. A patterned beam 17 is formedthat is imaged by projection system PS via reflective elements 18,19onto wafer stage or substrate table WT. More elements than shown maygenerally be present in the illumination system IL and projection systemPS.

One of the reflective elements 19 may have in front of it a numericalaperture (NA) disc 20 having an aperture 21 therethrough. The size ofthe aperture 21 determines the angle α_(i) subtended by the patternedradiation beam 17 as it strikes the substrate table WT.

In other embodiments, the radiation collector is one or more of acollector configured to focus collected radiation into the radiationbeam emission aperture; a collector having a first focal point thatcoincides with the source and a second focal point that coincides withthe radiation beam emission aperture; a normal incidence collector; acollector having a single substantially ellipsoid radiation collectingsurface section; and a Schwarzschild collector having two radiationcollecting surfaces.

Also, in another embodiment, the radiation source SO may be a laserproduced plasma (LPP) source including a light source that is configuredto focus a beam of coherent light, of a predetermined wavelength, onto afuel.

For example, FIG. 3 shows an embodiment of a radiation source unit 3, incross-section, including a normal incidence collector 70. The collector70 has an elliptical configuration, having two natural ellipse focuspoints F1, F2. Particularly, the normal incidence collector includes acollector having a single radiation collecting surface 70 s having thegeometry of the section of an ellipsoid. In other words: the ellipsoidradiation collecting surface section extends along a virtual ellipsoid(part of which is depicted by as dotted line E in the drawing).

As will be appreciated by the skilled person, in case the collectormirror 70 is ellipsoidal (i.e., including a reflection surface 70 s thatextends along an ellipsoid), it focuses radiation from one focal pointF1 into another focal point F2. The focal points are located on the longaxis of the ellipsoid at a distance f=(a2−b2)½ from the center of theellipse, where 2a and 2b are the lengths of the major and minor axes,respectively. In case that the embodiment shown in FIG. 1 includes anLPP radiation source SO, the collector may be a single ellipsoidalmirror as shown in FIG. 3, where the light source SO is positioned inone focal point (F1) and an intermediate focus IF is established in theother focal point (F2) of the mirror. Radiation emanating from theradiation source, located in the first focal point (F1) towards thereflecting surface 70 s and the reflected radiation, reflected by thatsurface towards the second focus point F2, is depicted by lines r in thedrawing. For example, according to an embodiment, a mentionedintermediate focus IF may be located between the collector and anillumination system IL (see FIGS. 1, 2) of a lithographic apparatus, orbe located in the illumination system IL, if desired.

FIG. 4 schematically shows a radiation source unit 3′ in accordance withan embodiment of the invention, in cross-section, including a collector170. In this case, the collector includes two normal incidence collectorparts 170 a, 170 b, each part 170 a, 170 b preferably (but notnecessarily) having a substantially ellipsoid radiation collectingsurface section. Particularly, the embodiment of FIG. 4 includes aSchwarzschild collector design, preferably consisting of two mirrors 170a, 170 b. The source SO may be located in a first focal point F1. Forexample, the first collector mirror part 170 a may have a concavereflecting surface (for example of ellipsoid or parabolic shape) that isconfigured to focus radiation emanating from the first focal point F1towards the second collector mirror part 170 b, particularly towards asecond focus point F2. The second mirror part 170 b may be configured tofocus the radiation that is directed by the first mirror part 170 atowards the second focus point F2, towards a further focus point IF (forexample an intermediate focus). The first mirror part 170 a includes anaperture 172 via which the radiation (reflected by the second mirror 170b) may be transmitted towards the further focus point IF. For example,the embodiment of FIG. 4 may beneficially be used in combination with aDPP radiation source.

In the present embodiment, the source SO is a LPP source, that isassociated with a laser source configured to generate a laser beam ofcoherent light, having a predetermined wavelength. The laser light isfocused onto a fuel (the fuel for example being supplied by a fuelsupplier, and for example including fuel droplets) to generate radiationthere-from, in a laser produced plasma process. The resulting radiationmay be EUV radiation, in this embodiment. In a non-limiting embodiment,the predetermined wavelength of the laser light is 10.6 microns (i.e.,μm). For example, the fuel may be tin (Sn), or a different type of fuel,as will be appreciated by the skilled person.

The radiation collector 70 may be configured to collect radiationgenerated by the source, and to focus collected radiation to thedownstream radiation beam emission aperture 60 of the chamber 3.

For example, the source SO may be configured to emit divergingradiation, and the collector 70 may be arranged to reflect thatdiverging radiation to provide a converging radiation beam, convergingtowards the emission aperture 60 (as in FIGS. 3 and 4). Particularly,the collector 70 may focus the radiation onto a focal point IF on anoptical axis O of the system (see FIG. 2), which focal point IF islocated in the emission aperture 60.

The emission aperture 60 may be a circular aperture, or have anothershape (for example elliptical, square, or another shape). The emissionaperture 60 is preferably small, for example having a diameter less thanabout 10 cm, preferably less than 1 cm, (measured in a directiontransversally with a radiation transmission direction T, for example ina radial direction in case the aperture 60 has a circularcross-section). Preferably, the optical axis OX extends centrallythrough the aperture 60, however, this is not essential.

Because infrared radiation (“IR”) that may be produced by the radiationsource SO may cause heating of the mirrors downstream of the collector,as well as the reticle stage, it is desirable to filter the IR from thedesired EUV radiation being provided to the patterning device MA. It mayalso be desirable to filter deep ultraviolet (“DUV”) radiation (forexample having a wavelength in a range of about 190-250 nm) from theEUV, because DUV may cause blurring of the EUV image in the resist onthe substrate W.

Therefore, a spectral purity filter may be provided within the radiationbeam path within the lithographic apparatus. Such a spectral purityfilter may be arranged, for example, to pass EUV radiation but block orre-direct radiation of other wavelengths, in particular wavelengths thatmay be undesirable.

It will be appreciated that such a spectral purity filter may not fullyblock or re-direct radiation of an undesirable wavelength. Furthermore,a spectral purity filter may not be able to block or re-direct radiationin different ranges of undesirable wavelengths. Furthermore, spectralpurity filters may undesirably reduce the intensity of the EUV radiationwithin the beam of radiation. Furthermore, a spectral purity filter maydegrade or fail after a given period of time due to the conditions underwhich it may operate. For example, due to the overall intensity of theradiation within the beam of radiation, significant heating of thespectral purity filter may occur. Furthermore, the beam of radiation maybe pulsed, which may result in fluctuating temperatures of somearrangements of spectral purity filters, including significant physicalstress.

It has been realized that no single spectral purity filter is capable ofmeeting all of the desired requirements for a lithography apparatus,namely sufficient attenuation of undesired radiation wavelengths,including multiple different ranges of undesired wavelengths, sufficienttransmission of desired wavelengths and sufficient durability within alithographic apparatus. However, it has previously also been undesirableto include multiple spectral purity filters of any of the knownarrangements due to the consequent compound attenuation of the desiredradiation wavelength, such as EUV radiation. Attenuation of the desiredradiation wavelength in the beam of radiation in a lithographicapparatus is undesirable because it may increase the time taken to forma desired pattern on a substrate, resulting in reduced throughput of theapparatus.

According to an embodiment of the present invention, two differentarrangements of spectral purity filter are used within a lithographicapparatus. Beneficially, by using two different arrangements of spectralpurity filter, it is possible to compensate for deficiencies of one formof spectral purity filter by the use of a different form of spectralpurity filter.

For example, as is discussed in further detail below, the use of a firstform of spectral purity filter may reduce the heating of a secondspectral purity filter, enabling the selection of parameters for thesecond spectral purity filter to be optimized for, for example,filtering of a particular wavelength of radiation, with fewerconstraints on the durability of the second filter. Accordingly, theremay be significant benefits from selecting two different types orarrangements of spectral purity filter in comparison to providing alithographic apparatus with two spectral purity filters of a singledesign.

In embodiments of the present invention, the two different spectralpurity filters may be selected from at least grating spectral purityfilters, membrane spectral purity filters, grid spectral purity filtersand anti reflection coatings, as discussed below. It should beappreciated that more than two different types of spectral purity filtermay also be used.

Non-limiting embodiments of a grating spectral purity filter 11 areillustrated in FIG. 5 and are represented as A, B, C, and D. Asdiscussed in further detail below, the grating spectral purity filtermay be provided on a mirror of any of the collectors discussed above, toa mirror in the illumination system IL, such as mirrors 18 or 19 shownin FIG. 2 or any reflector in the lithography apparatus.

It is desirable, although not necessary, for the spectral purity filter11 to fulfill the following specification:

Absorbed + After SPF [W] Reflected power at Region At IF [W](transmission) SPF [W] in-band EUV 130 >105 <25 OoB EUV 9-21 nm 100<80 >20 OoB EUV 5-130 nm 250 <200 >60 DUV 130-400 nm 200 <2 >198 VIS-NIR10 <5 >5 IR 10 <1 >9 10.6 um 260 <26 >234

In order for the collector to remain reflective for radiation having awavelength of 13.5 nm (EUV), the grating spectral purity filter 11includes a coating that is applied to a smooth (e.g., polished)substrate that is typically used for the collector. The coating maycomprise a plurality of layers (see FIG. 5) that alternate in materialso as to create a so-called multilayer stack 100 on the smoothsubstrate. In an embodiment, the multilayer stack 100 may include aplurality of alternating layers in the order of about 1000 and have atotal thickness of about 7 μm. Any suitable combination of materials forthe alternating layers that are known in the art may be used.

After the multilayer stack 100 has been applied to the smooth substrate,a top side of the multilayer stack may be etched, mechanically processedusing, for example, diamond turning or sputtered away in, for example, arandom square (see A in FIG. 5), a random saw (see D in FIG. 5), or arandom wave pattern (see B and C in FIG. 5) to create a plurality ofrecesses 110 in the top side of the multilayer stack 100 thereby formingthe grating spectral purity filter 11.

In an embodiment, the recesses 110 may have a symmetric cross-section,for example as shown in FIG. 5.

In an embodiment, the recesses 110 may have about a depth of aboutone-fourth of the wavelength of the undesired radiation, i.e., □/4, andsuitable profile see FIG. 5) that allow the recesses to either scatter(about 50×), or reflect the 0 order of the undesired radiation (e.g., IRand/or DUV) in a direction that is different than the direction that thedesired EUV radiation is reflected. At the same time, the EUV contrastmay be determined by the plurality of alternating layers in themultilayer stack, as is known in the art. The desirable EUV radiationmay be reflected to the intermediate focus IF, either directly, or withthe use of additional mirrors.

The grating spectral purity filter 11 of FIG. 5 is configured to enhancea spectral purity of the radiation that is to be emitted via theaperture 60 (shown in FIGS. 3 and 4). In an embodiment, the filter 11 isconfigured to transmit only a desired spectral part of radiation towardsthe aperture 60. For example the filter 11 may be configured to reflect,block, or redirect other ‘undesired’ spectral parts of the radiation.Preferably, the filter 11 is configured to provide a combination of oneor more of blocking, redirecting and reflecting other ‘undesired’spectral parts of the radiation.

In accordance with an embodiment, a desired spectral part (i.e., to beemitted via the aperture 60) is FIN radiation (for example having awavelength lower than 20 nm, for example a wavelength of 13.5 nm). Thefilter 11 may be configured to transmit at least 50%, preferably morethan 80%, of incoming radiation (i.e., radiation that is directedtowards the filter from the source SO) of that desired spectral part.For example, to filter out radiation having a wavelength λ of about 10μm, the recesses in the top side of the multilayer stack may be about2.5 μm deep.

In an embodiment, the spectral purity filter may also include a thincoating that is provided to the top side of the multilayer stack afterthe recesses have been created. The coating may have a thickness ofabout 0.2 nm to about 1 nm. The coating may include a metal thatexhibits high electrical conductivity and does not oxidize. For example,the metal may be selected from the group consisting of Ru, Pd, Pt, Rh,Ro, Ti, Au, Mo, Zr, Cu, Fe, Cr, Ni, Zn, and Ag. In an embodiment, themetal may be selected from the group consisting of Ru, Pd, Pt, Rh, Ro,Ti, and Au.

In an embodiment, a reflective multilayer stack may be deposited to apolished collector mirror. The reflective multilayer stack may beprovided with a grating spectral purity filter by wet etching, dryetching, scratching or other mechanical process and/or using anysuitable lithographic techniques to transfer the desired spectral purityfilter to the reflector surface.

In an embodiment, the reflective multilayer stack may be deposited ontoa substrate and the grating spectral purity filter, formed as above. Thesubstrate, including the multilayer reflector and grating spectralpurity filter may then be attached to the polished collector mirrorwith, for example, a suitable adhesive.

FIGS. 8a to 8g depict a process by which a grating spectral purityfilter for use in the present invention may be formed. As shown, theprocess commences with a substrate 300, which may, for example, be thepolished collector mirror.

A reflective multilayer stack 301 is formed on a surface of thesubstrate 300. A layer of radiation sensitive material, such as a resist302, is deposited on top of the reflective multilayer stack 301. Aninterference pattern of the radiation 303 is then projected onto theradiation sensitive layer 302. The material of the radiation sensitivelayer 301 is then developed in order to produce a patterned mask 304 onthe surface of the reflective multilayer stack 301. The surface is thenetched, for example, chemically etched, such that the patterned mask 304produces, under the influence of the etch, a textured surface, 305 onthe surface of the reflective multilayer stack 301, forming the spectralpurity filter. Finally, if required, a thin coating 306 may be formed onthe topside of the multilayer stack, as discussed above.

It will be appreciated that, as discussed above, the substrate 300 maybe the component on which it is desired to form the grating spectralpurity filter, such as a reflector. Alternatively, the substrate may bea separate component on which the spectral purity filter is formed andwhich is then attached to the component on which the spectral purityfilter is desired to be located.

FIG. 9 schematically depicts a system that may be used to project aninterference pattern of radiation onto the substrate during theformation of a grating spectral purity filter using the method depictedin FIGS. 8a to 8g . It will be appreciated, however, that alternativesystems may be provided in order to project an interference pattern ofradiation onto the surface on which the grating spectral purity filteris to be formed.

As shown in FIG. 9, the system may include a narrow band source ofradiation 350. For example, the narrow band source of radiation 350 mayinclude a UV source 351 and a narrow band filter 352. The system furtherincludes an arrangement for introducing an interference pattern into thebeam of radiation produced by the narrow band source of radiation 350.For example, as depicted in FIG. 9, an etalon 353 may be provided (alsoknown as a Fabry-Pérot interferometer). In addition, optical componentsmay be provided in order to appropriately project the interferencepattern onto the target 354. For example, as shown, asphericalbeam-expander optics 355 may be provided between the radiation source350 and the etalon 353 and a field lens 356 may be provided between theetalon and the target 354.

It will be appreciated that if an arrangement such as that depicted inFIG. 9 is used, the interference pattern of radiation projected onto thetarget 354 may be adjusted by adjustment of the wave length of radiationused, the intensity of the beam of radiation and/or by adjusting theetalon spacing.

It should also be appreciated that the method of forming a gratingspectral purity filter as discussed above, in particular using anarrangement such as depicted in FIG. 9 to project an interferencepattern of radiation onto a substrate, may enable the formation of therequisite textured surface on a relatively large component, such as acollector mirror as discussed above. Furthermore, such a system may bebeneficial because it may be applicable to the formation of a texturedsurface, and therefore the formation of a spectral purity filter asdiscussed above, on a curved surface.

According to an aspect of the invention, the lithographic apparatus mayuse two grating spectral purity filters, each arranged with differentradiation filtering structures from each other.

In an embodiment, the two different grating spectral purity filters maybe provided on different reflectors within the lithographic apparatus.Accordingly, both may be formed from a multilayer stack having aplurality of recesses formed in the top surface, as discussed above. Forboth grating spectral purity filters, the multilayer stack may beconfigured such that radiation of a first wavelength, such as EUVradiation, is reflected in a first direction relative to the multilayerstack. The recesses may be configured such that radiation of a differentwavelength from the first wavelength is reflected in a differentdirection from the first direction in order to filter undesiredradiation wavelengths.

In an embodiment of the present invention, the recesses of the firstgrating spectral purity filter have a different dimension from therecesses of the second grating spectral purity filter, namely adifferent radiation filtering structure. Accordingly, the two gratingspectral purity filters may be used to filter different wavelengths ofradiation from the optical path. For example, one grating spectralpurity filter may be configured in order to suppress infrared radiation,for example having a wavelength of approximately 10.6 μm and the secondgrating spectral purity filter may be configured to suppress or removeDUV radiation.

In an embodiment of the present invention, both the first and the secondspectral purity filters may be provided together on one reflector withinthe lithographic apparatus. FIGS. 10 and 11 schematically depict twoarrangements of possible combinations of two grating spectral purityfilters. As shown, as with the grating spectral purity filters discussedabove, the grating spectral purity filters depicted in FIGS. 10 and 11are formed from a multilayer stack 100, in which a first plurality ofrecesses 110 are formed. As with the spectral purity filters discussedabove, the multilayer stack 100 may be configured to reflect radiationof a first wave length in a first direction relative to the multilayerstack 100 and the first plurality of recesses 110 may be configured toreflect radiation of a second wavelength in a second direction relativeto the multilayer stack 100, that is different from the first direction.

In addition to the plurality of first recesses 110, a second pluralityof recesses 120, 121 are formed. As shown in FIGS. 10 and 11, the secondplurality of recesses 120, 121 are smaller than the first plurality ofrecesses 110 and are formed on the remaining top surface 100 a of themultilayer stack 100, between the recesses 110 of the first plurality ofrecesses, and on the lower surface 110 a of each of the recesses 110.

The second plurality of recesses 120, 121 may be configured to reflect athird wavelength of radiation, different from the first and secondwavelengths of radiation, in a direction that is also different from thefirst direction.

Accordingly, for example, the multilayer stack 100 may be used totransmit radiation of the first wavelength along the optical path of thelithographic apparatus, the first plurality of recesses 110 may be usedto remove or suppress a second radiation wavelength in the optical pathand the second plurality of recesses 120, 121 may be used to remove orsuppress a third radiation wavelength from the optical path.

As with the grating spectral purity filters depicted in FIG. 5, thefirst and second pluralities of recesses may have any desiredcross-section and may, in particular, have a symmetrical cross-section.In the examples depicted in FIGS. 10 and 11, the first plurality ofrecesses 110 may have a rectangular cross section, also referred to as“binary gratings”. An advantage of a rectangular cross-section for thefirst, larger, plurality of recesses 110 is that they may be formed by amechanical process, such as diamond turning. This may reduce the cost ofproducing a reflector having combined first and second grating spectralpurity filters.

In the example depicted in FIG. 10, the second plurality of recesses 120also have a rectangular cross-section. An advantage of such anarrangement is that the majority of the exposed surface of themultilayer stack may be a single layer of the multilayer stack.Accordingly, if the multilayer stack is formed from two alternatinglayers, one of which is more susceptible to degradation in theenvironment in which the reflector is to be used than the other, thecombined grating spectral purity filter may be configured such that thetop layer, which is primarily exposed, is formed from the one of the twomaterials of the multilayer stack that is least susceptible todegradation. Accordingly, the lifetime of the combined grating spectralpurity filter may be increased.

In the example depicted in FIG. 11, the second plurality of recesses 121are blazed, namely have a triangular cross-section. An advantage ofusing a blazed grating is that the grating may reflect a broader rangeof wavelengths in the first direction, and therefore be used to suppressor remove those wavelengths from the optical path, than is possibleusing a binary grating. This may be particularly beneficial if it is toremove or suppress DUV radiation, which may be present across asignificant range of wavelengths.

As is discussed above, selection of the depth of the recesses 110, 120,121 may be used to select the radiation wavelength that the two gratingspectral purity filters remove or suppress from the optical path of thelithographic apparatus. The pitch of the recesses 110, 120, 121 may beused to control the divergence between the first direction, namely thedirection of radiation that is reflected by the multilayer stack 100,and the second and third directions, namely the direction of theradiation reflected by the first and second pluralities of recesses 110,120, 121. Accordingly, the undesired radiation may effectively form acone surrounding the optical axis of the beam of radiation, the desiredradiation remaining on axis. Accordingly, if the beam of radiation isdirected through an aperture from the grating spectral purity filter,for example at a point of intermediate focus between the radiationsource and the illumination system, the undesirable radiation may notpass through the aperture but may be incident on, and absorbed by, awall of the system, such as a wall of the evacuation chamber in whichthe source and/or illumination system may be located.

If desired, the pitch of the two pluralities of recesses may be selectedsuch that the second and third directions are the same. In such anarrangement, both radiation wavelengths being removed or suppressed fromthe optical path may be reflected out of the optical path of thelithographic apparatus in a common direction to a common radiation sink.

Alternatively, the second and third directions may be different andseparate radiation sinks may be provided for the two radiationwavelengths being suppressed or removed from the optical path.

In an embodiment, the first plurality of recesses 110 may have a depthof between approximately 1.5 μm to 3 μm, desirably approximately 2.65μm. The pitch of the first plurality of recesses 110 may beapproximately 2 mm. Such an arrangement may be used to remove orsuppress infrared radiation from the optical path of the lithographicapparatus, in particular radiation of a wavelength of 10.6 μm.

The second plurality of recesses 120, 121 may, for example, have a depthof between approximately 25 nm to 75 nm, desirably approximately 50 nmand may have a pitch of, for example 0.04 mm. Accordingly, the secondplurality of recesses 120, 121 may form a second grating spectral purityfilter suitable for removing or suppressing DUV radiation from theoptical path of the lithographic apparatus.

According to an embodiment, a grating spectral purity filter asdiscussed above may be configured to filter at least part of radiationhaving the predetermined wavelength of the coherent laser light from theradiation source, from the radiation that is to be emitted.Particularly, a desired part of radiation that is to be emitted has asignificantly lower wavelength than the coherent laser light. Thewavelength of the coherent laser light may be, for example, larger than10 microns. In an embodiment, the coherent laser light, to be filteredout, has a wavelength of 10.6 microns.

In the above, a grating spectral purity filter has been applied inradiation systems, including a radiation collector. In an embodiment, agrating spectral purity filter of any of the arrangements discussedabove may be applied to mirrors in the illumination system IL of thelithographic apparatus or other reflectors in a lithographic apparatus.

By combining a grating spectral purity filter with the collector mirror,the out of band radiation may be dealt with closer to its source so thatno additional EUV (or minimal) loss is realized due to implementation offiltering techniques more upstream of the so-called optical column.Because the grating spectral filter is positioned at the largest surfacein the optical column, it may have relatively low power loads.Furthermore, a long optical path until the intermediate focus IF isavailable, which may allow for small diverting angles to be used toprevent the unwanted radiation from leaving the source SO and enteringthe illuminator IL. The grating spectral purity filter of embodiments ofthe present invention may remain working in a hostile environment and asa result may not need to be replaced by costly filters, therebypotentially saving money.

FIG. 6 depicts schematically a part of an illumination system that maybe provided for use in a lithographic apparatus according to anembodiment of the present invention. In particular, the arrangementdepicted in FIG. 6 may be provided in order to at least partiallycondition a beam of radiation.

As shown in FIG. 6, radiation is provided from a point of intermediatefocus 200 to a first array of reflectors 201, which each focus a portionof the beam of radiation onto a respective reflecting element in asecond array of reflecting elements 202. Each of the reflecting elementsin the second array of reflecting elements 202 is configured to directthe radiation incident on the reflector of the second array ofreflectors 202 into a conditioned beam of radiation 203 to be providedby the radiation beam conditioning device.

The conditioned beam of radiation 203 may be, for example, directed ontoa patterning device that is used to impart a pattern to the beam ofradiation as part of the lithography process. In such an arrangement,each of the reflectors in the second array, of reflectors 202 may beconfigured such that the field of the associated element in the firstarray of reflectors 201 is imaged onto the patterning device. Such anarrangement is commonly known as a “fly's eye integrator”. In such anarrangement, the reflectors of the first array of reflectors 201 arecommonly referred to as field facet mirrors and the reflectors of thesecond array of reflectors 202 are commonly referred to as pupil facetmirrors. As will be appreciated, such an arrangement is configured suchthat the field at the patterning device (or at the outlet of theradiation beam conditioning device) consists of a plurality ofoverlapping images of the first array of reflectors 201. This provides amixing of the radiation from the point of intermediate focus 200, namelyof the radiation emitted by a radiation source, providing improvedillumination uniformity.

In the arrangement depicted in FIG. 6, each of the reflectors of thefirst array of reflectors 201 is provided with a grating spectral purityfilter according to one of the arrangements discussed above.

Furthermore, the first and second arrays of reflectors 201,202 areconfigured such that radiation having the desired wavelength isreflected from each reflector of the first array of reflectors 201 to arespective reflector of the second array of reflectors 202. Therespective reflectors of the second array of reflectors 202 areappropriately configured to reflect the radiation of the desiredwavelength to form a part of the conditioned beam of radiation 203.Radiation of undesired wavelengths, on the other hand, is reflected fromeach reflector of the first array of reflectors 201 in a differentdirection and therefore incident on a different reflector of the secondarray of reflectors 202. In this case, the radiation of the undesiredwavelength reflects from the reflector of the second array of reflectors202 on which it is incident in such a manner that it does not form apart of the conditioned beam of radiation 203.

In a preferred arrangement, as depicted in FIG. 6, a radiation absorber204 is provided that is configured to absorb the radiation of theundesired wavelengths that is reflected in a direction such that it doesnot form part of the conditioned beam of radiation 203. For example, theradiation absorber 204 may be arranged in the form of an aperture thatpermits radiation of the desired wavelength that has been reflected in afirst direction, to pass through the aperture to form the conditionedbeam of radiation 203 but absorbs radiation of the undesired wavelengththat is reflected in a different direction.

It should be appreciated that in an arrangement such as that depicted inFIG. 6, radiation having undesired wavelengths may be reflected by eachof the reflectors of the first array of reflectors 201 onto a pluralityof reflectors of the second array of reflectors 202. In addition, eachof the reflectors of the second array of reflectors 202 may both receiveradiation from a first reflector in the first array of reflectors 201having the desired wavelength, reflecting such desired radiation suchthat it is included in the conditioned beam of radiation 203, andreceive radiation from one or more other reflectors of the first arrayof reflectors 201 that has an undesired wavelength, reflecting suchundesired radiation such that it does not form part of the conditionedbeam of radiation 203.

FIG. 7 schematically depicts an arrangement that is similar to thearrangement depicted in FIG. 6. Accordingly, description ofcorresponding features will be omitted for brevity. The differencebetween the arrangement depicted in FIG. 7 and the arrangement depictedin FIG. 6 is that the first and second arrays of, reflectors 201,202 areconfigured such that the radiation of undesired wavelengths is reflectedfrom each reflector of the first array of reflectors 201 such that theradiation is directed to a space between two of the reflectors of thesecond array of reflectors 202. Accordingly, only radiation of thedesired wavelength is reflected by the reflectors of the first array ofreflectors 201 onto the reflectors of the second array of reflectors 202and subsequently form part of the conditioned beam of radiation 203. Asshown in FIG. 7, a radiation absorber 205 may be provided on theopposite side of the second array of reflectors 202 from the first arrayof reflectors 201. The radiation absorber 205 may be configured toabsorb the radiation of the undesired wavelengths that passes betweenthe reflectors of the second array of reflectors 202.

The radiation absorbers 204;205 of the arrangements depicted inarrangements depicted in FIGS. 6 and 7 may be provided with a coolingsystem in order to dissipate the heat resulting from the absorption ofthe radiation of the undesired wavelengths.

It should also be appreciated that, although the arrangements depictedin FIGS. 6 and 7 are such that the grating spectral purity filters areprovided on the reflectors of the first array of reflectors 201,alternative arrangements may be provided in which the grating spectralpurity filters are alternatively or additionally provided on thereflectors of the second array of reflectors 202. In either case, thegrating spectral purity filters may be arranged such that radiation ofthe desired wavelength is directed such that it forms the conditionedbeam of radiation while radiation having undesired wavelengths isdirected in one or more different directions and may be absorbed by anappropriate radiation absorber.

The arrangements of FIGS. 6 and 7 may beneficially prevent undesiredwavelengths of radiation from passing into the remainder of alithographic apparatus. In addition, it may be easier to form thegrating spectral purity filters on the reflectors of the arrays ofreflectors used in the radiation beam conditioning system than it is toform the grating spectral purity filter on, for example, the collectormirrors as discussed above. Furthermore, the environment within whichthe collector mirrors operate may be such that the useful lifetime ofthe grating spectral purity filter formed on the collector mirror may beshorter than the useful lifetime of the grating spectral purity filterwhen formed on the reflectors of the radiation beam conditioning systemdepicted in FIGS. 6 and 7.

An example of a membrane spectral purity filter 130 is depicted in FIG.12. As shown, the membrane spectral purity filter 130 is formed from athin film of material 131 that may be supported by a frame 132. The beamof radiation 133 to be filtered is directed at, and passes through, thethin film of material 131. The thin film of material 131 is selected tobe as transparent as possible to the desirable wavelengths of radiationand not transparent to the undesirable radiation wavelength. Forexample, the thin film 131 may be formed from layers of Zr and Si. Thethin film may, for example, have a thickness in the range from about 50nm to approximately 250 nm.

In order to support the thin film 131, a supporting mesh may beprovided. The mesh may increase the durability of the membrane spectralpurity filter 130 but may reduce its transparency to the desirableradiation wavelengths. It will be appreciated that the present inventionmay be implemented with any presently known membrane grating spectralpurity filter.

The membrane grating spectral purity filter 130 may be used at any of avariety of locations within the lithographic apparatus. However, it maybe preferable to locate the membrane spectral purity filter 130 atlocations selected such that the beam of radiation only passes throughthe thin film or material 131 once. This is because, although the thinfilm of material 131 is selected to be as transparent as possible to thedesirable radiation wavelengths, such as EUV radiation, each time thebeam of radiation 133 passes through the membrane spectral purity filter130, the intensity of the desirable radiation wavelength will beattenuated. The compound effect of passing the beam of radiation throughthe thin film of material 131 may therefore result in unacceptableattenuation of the intensity of the desirable wavelength of radiation.Therefore, for example it may not be appropriate to use the membranespectral purity filter 130 adjacent a folding mirror within thelithographic apparatus, at which the optical path of the beam ofradiation is substantially bent back upon itself.

In an embodiment, it may be desirable to arrange a membrane spectralpurity filter 130 close to the point of intermediate focus 12 discussedabove, namely the virtual point source that may be provided in the beamof radiation for entry into the illumination system IL. Accordingly, forexample, the membrane spectral purity filter 130 may be provided as thelast element before the intermediate focus point 12 or the first elementafter the intermediate focus point 12. In either case, it will beappreciated that the membrane spectral purity filter 130 may be providedas part of the radiation system or as part of the illumination system.Such an arrangement may be beneficial because the location is providedin the optical path of lithographic apparatus relatively close to theradiation source. In general, the closer to the radiation source alongthe optical path, the wider the beam of radiation and therefore thelower the radiation intensity incident on a given area of the thin filmof material 131.

In an embodiment of the present invention, the illumination system ILmay include a grazing incidence reflector. In particular, the grazingincidence reflector may be the last optical element in the illuminator.The membrane spectral purity filter 130 may be arranged adjacent to thegrazing incidence reflector because at such a location it may beconfigured such that the beam of radiation only passes through the thinfilm of material 131 once.

Accordingly, for example, the membrane spectral purity filter 130 may bearranged as the last element in the optical path before the grazingincidence reflector or the first element in the optical path after thegrazing incidence reflector.

The membrane spectral purity filter may be arranged such that thesurface of the thin film of material 131 is substantially perpendicularto the optical axis of the beam of radiation. Alternatively, it may bearranged to be at an oblique angle. For example, if the membranespectral purity filter 30 is arranged adjacent the intermediate focuspoint 12, it may be arranged at an angle of approximately from 1 to 30°C., for example approximately 15° C. If the membrane spectral purityfilter 130 is adjacent the grazing incidence reflector, it may bearranged at an angle of approximately from 20 to 65° C.

It will be appreciated that other locations and/or arrangements for themembrane spectral purity filter may also be considered.

An example of a grid spectral purity filter 140 is depictedschematically in FIG. 13. As shown, the grid spectral purity filter 140may be formed from one or more apertures 141 formed in a substrate 142.The one or more apertures 141 may, as depicted in FIG. 13 be an array ofpinholes. However, this is not essential and the aperture(s) may haveany appropriate cross-section including, for example, a slit of a givenwidth. Each aperture 141 reflects substantially all radiation withwavelengths for which the aperture width is below the diffraction limit,the diffraction limit being half the wavelength in the medium that fillsthe aperture 141 (which may be vacuum). For aperture widths above thediffraction limit, a substantial fraction of the radiation istransmitted through the aperture.

As an example, for a slit with a 100 nm width, substantially all lightwith wavelengths larger than 200 nm is reflected.

Accordingly, by appropriate selection of the width of the one or moreapertures 141, the grid spectral purity filter 140 may be configured totransmit a desired radiation wavelength, such as EUV radiation andreflect radiation having a longer wavelength.

It will be appreciated that the transparency of the grid spectral purityfilter 40 to the desired wavelength of radiation will be dependent onthe proportion of the area on which the beam of radiation 143 isincident that is open. Accordingly, because the width of the one or moreapertures 141 may be small, in particular if it is desirable to betransparent to EUV radiation, a plurality of apertures 141 may beprovided, as depicted in FIG. 13.

Although FIG. 13 depicts a periodic array of apertures 141, any suitablearray forming a regular or irregular pattern may be used. In certaincircumstances, it may be desirable to vary the spacing between theapertures 141 in order to avoid unwanted diffraction effects due to theperiodicity of constant spacing between the apertures 141.

The apertures 141 may be formed using lithographic and/ormicro-machining techniques. For example, a micro-machining techniqueinvolves defining apertures in a layer on top of a silicon wafer byphotolithography followed by etching deep into the silicon wafer. Inorder to open the apertures 141, a window is etched into the back sideof the silicon wafer, for example by using KOH etching techniques. Itwill be appreciated, however, that any known grid spectral purity filtermay be used in the present invention.

A grid spectral purity filter 140 may be used at any desired locationwithin the lithographic apparatus. However, as with the membranespectral purity filter discussed above, it may be preferable to locatethe grid spectral purity filter at a position in which the beam ofradiation 143 only passes through the grid spectral purity filter 140 ina single direction. As with the membrane spectral purity filter 130,this may reduce unwanted attenuation of the desirable radiationwavelength in the beam or radiation.

Furthermore, it may be desirable to locate the grid spectral filter 140at a location that is relatively close to the radiation source becausethe attenuation of the desirable radiation wavelength is greater if theradiation is incident on an aperture 141 at an angle.

In an embodiment, the grid spectral purity filter 140 may be locatedclose to the point of intermediate focus 12 discussed above provided forthe entry of the beam or radiation into the illumination system IL. Forexample, as with the membrane spectral purity filter 130, the gridspectral purity filter 130 may be provided as the final element beforethe point of intermediate focus 12 or as the first component after thepoint of intermediate focus 12. Likewise, the grid spectral purityfilter 140 may be provided as part of the radiation system or part ofthe illumination system.

FIG. 14 schematically depicts an arrangement of an anti reflectioncoating 150 that may be used as a spectral purity filter in the presentinvention. As depicted, it may be formed from a thin film of material151 formed on the surface of a reflector 152 within the lithographicapparatus. It will be appreciated that the thin film 151 may be providedon any one of the reflectors within the lithographic apparatus.Furthermore, it may be provided on a plurality of the reflectors withinthe lithographic apparatus. The thin film 151 may be selected such thatit substantially does not affect the reflective properties of thereflector 152 for the desirable radiation wavelength of the beam orradiation 153 but inhibits reflection of one or more undesirableradiation wavelengths.

The thin film 151 may be formed from a dielectric coating, for exampleformed from silicon nitride, silicon oxide or any other suitablematerial. It will be appreciated that any known anti reflection coatingsmay be used as spectral purity filters in the present invention.

As described above, in an embodiment of the present invention, twodifferent spectral purity filters are used. The combination of twodifferent spectral purity filters may provide particular benefits.

For example, it may be desirable to provide a grating spectral purityfilter on the collector, as discussed above. However, this may notprovide sufficient removal of the infrared radiation. In particular, forexample, an arrangement of a grating spectral purity filter may reducethe intensity of the infrared radiation within the beam of radiation toapproximately 2% of its initial value but it may be desirable to havethe infrared radiation reduced to 0.02% of its initial value.

Accordingly, in an embodiment, the lithographic apparatus may include agrating spectral purity filter in combination with a membrane spectralpurity filter. For example, a membrane spectral purity filter may reducethe infrared radiation in a beam of radiation to 1% of the receivedintensity. Therefore, the combination of both spectral purity filters incombination may reduce the intensity of the infrared radiation to 0.02%of its initial intensity.

Using these two different spectral purity filters provides additionalbenefits. In particular, for example, a single grating spectral purityfilter configured to remove infrared radiation may not reduce theintensity of the DUV radiation in the radiation beam. However, themembrane spectral purity filter may provide the require reduction in DUVradiation.

Furthermore, a problem with some configurations of EUV lithographicapparatus is the transmission of Sn particles from the radiation sourceinto the remainder of the apparatus. These may degrade the performanceof the optical components within the lithographic apparatus or, morecritically, may damage a patterning device. The provision of a membranespectral purity filter, for example adjacent to the point ofintermediate focus 12, as discussed above, may significantly reduce thetransmission of Sn particles without significant reduction in theproperties of the membrane spectral purity filter. However, it is notalways possible to provide a membrane spectral purity filter due to thelack of durability of membrane spectral purity filters. However, the useof a grating spectral purity filter may effectively protect the membranespectral purity filter, permitting its use.

The provision of the grating spectral purity filter may, in particular,permit the use of the membrane spectral purity filter in a desirableform and/or at a desirable location. For example, without the gratingspectral purity filter, particularly located closer to the radiationsource than the membrane spectral purity filter, the heat load on themembrane spectral purity filter may be too large, either preventing itsuse at a particular location or resulting in an unacceptably shortperiod of use before the membrane spectral purity filter would need tobe replaced. The provision of the grating spectral purity filter mayvery significantly reduce the heat load on the membrane spectral purityfilter.

Furthermore, by reducing the load of infrared radiation on the membranespectral purity filter, it may be made from materials that arerelatively absorbing of infrared. This, in turn, means that it will haveincreased emissivity at infrared and so will cool more quickly, whichmay be beneficial.

Furthermore, the provision of the grating spectral purity filter maysufficiently reduce the requirements of the membrane spectral purityfilter, in terms of the durability required of the membrane spectralpurity filter and/or the filtering effect of particular undesirableradiation wavelengths, that the design criteria of the membrane spectralpurity filter may be relaxed, providing further benefits. For example,reducing the durability requirements for the membrane spectral purityfilter may enable the use of a membrane spectral purity filter that doesnot include a supporting grid for the thin film of material. This mayincrease the transparency of the membrane spectral purity filter to thedesirable radiation wavelength, such as EUV radiation.

In an alternative embodiment, the lithographic apparatus may combine theuse of a grating spectral purity filter with a grid spectral purityfilter. As with the combination discussed above using the membranespectral purity filter, use of the grid spectral purity filter mayreduce the transmission of Sn particles into the remainder of theapparatus, in particular if the grid spectral purity filter is providedclose to the point of intermediate focus 12, as discussed above.

Furthermore, the use of the two different spectral purity filters mayreduce the requirements of one or both of the filters. For example, agrating spectral purity filter may reduce the power significantly ineither or both infrared and DUV. Accordingly, a subsequent grid spectralpurity filter may be thinner than it would need to be without thepresence of the grating spectral purity filter, for example 1 μm to 3μm, preferably 2 μm, presenting a lower angular variation of absorbanceof the desired radiation. This in turn, permits greater flexibility inselection of the location of the grid spectral purity filter.

However, the grid spectral purity filter may not reduce the transmissionof Sn particles into the remainder of the apparatus as effectively as amembrane spectral purity filter, as discussed above, because the gridspectral purity filter may only prevent transmission of relatively largeSn particles (for example, greater than approximately 3 to 5 μm).Furthermore, the combination of a single grating spectral purity filterand a grid spectral purity filter may not sufficiently suppress DUVradiation.

Therefore, for example, a combined grating spectral purity filter, asdiscussed above, may be used in combination with the grid spectralpurity filter, having a first grating spectral purity filter configuredto suppress infrared radiation and a second spectral purity filterprovided to suppress DUV radiation. Alternatively or additionally, ananti reflection coating may be provided on one or more of the reflectorswithin the lithographic apparatus in order to reduce the transmission ofDUV radiation.

It will be appreciated that other combinations of different spectralpurity filters may also be considered. For example, a lithographicapparatus may use a combination of a grating spectral purity filter andanti reflection coatings on one or more reflectors within thelithographic apparatus. Similarly, a lithographic apparatus may use agrid spectral purity filter in combination with a membrane spectralpurity filter.

Likewise, it should be appreciated that a lithographic apparatus may usethree of more spectral purity filters in combination. For example, alithographic apparatus may include a grating spectral purity filter, agrid spectral purity filter and a membrane spectral purity filter incombination. Such a combination may optionally also include the use ofantireflection coatings on one or more reflectors within thelithographic apparatus.

In general, it may be desirable for the first spectral purity filter tobe selected to significantly reduce the power of undesirable radiationin the beam, for example, attenuating infrared radiation. The secondspectral purity filter may be selected to reduce DUV radiation.

In an arrangement, one spectral purity filter may be provided in theradiation source and another outside the source, for example in theillumination system. The radiation source may, in some apparatus, beconsidered to terminate at a point of intermediate focus between thesource and the illumination system.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g., semiconductor memory, magnetic or optical disk) havingsuch a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

It is to be understood that in the present application, the term“including” does not exclude other elements or steps. Also, each of theterms “a” and “an” does not exclude a plurality. Any reference sign(s)in the claims shall not be construed as limiting the scope of theclaims.

What is claimed is:
 1. A lithographic apparatus configured to pattern a beam of radiation and project the beam of radiation onto a substrate, comprising: a first spectral purity filter configured to reduce the intensity of radiation in the beam of radiation in at least one first undesirable range of radiation wavelengths; and a second spectral purity filter configured to reduce the intensity of radiation in the beam of radiation in at least one second undesirable range of radiation wavelengths that is different than the at least one first undesirable range of radiation wavelengths; wherein the first spectral purity filter and the second spectral purity filter are provided with different radiation filtering structures from each other; wherein the first spectral purity filter is a grating spectral purity filter, comprising: a first multilayer stack of alternating layers, configured to reflect radiation of a first wavelength in a first direction relative to the first multilayer stack; and a first plurality of recesses in a top side of the first multilayer stack, the first plurality of recesses configured to form a grating arranged such that radiation of a second wavelength is reflected in a second direction relative to the first multilayer stack that is different from the first direction; and wherein the second spectral purity filter is a second grating spectral purity filter comprising: a second plurality of recesses, smaller than the first plurality of recesses, formed on the top side of the first multilayer stack between the recesses of the first plurality of recesses and in a lower surface of the recesses of the first plurality of recesses; and wherein the second plurality of recesses are configured such that radiation of a third wavelength is reflected in a third direction relative to the first multilayer stack that is different from the first direction.
 2. The lithographic apparatus of claim 1, wherein the first spectral purity filter is configured to reduce the intensity of infrared radiation in the beam of radiation; and the first spectral purity filter is arranged within the radiation beam path closer to a source of radiation that provides the beam of radiation than the second spectral purity filter is to the source of radiation.
 3. The lithographic apparatus of claim 1, wherein the lithographic apparatus further comprises a radiation system that provides the beam of radiation; and the first spectral purity filter is formed on a reflective surface of at least part of a collector in the radiation system.
 4. The lithographic apparatus of claim 1, wherein the first plurality of recesses have a depth of between approximately 1.5 μm to 3 μm.
 5. The lithographic apparatus of claim 4, wherein the depth is approximately 2.65 μm.
 6. The lithographic apparatus of claim 1, wherein the second plurality of recesses have a depth of between approximately 25 nm to 75 nm.
 7. The lithographic apparatus of claim 6, wherein the depth is approximately 50 nm.
 8. The lithographic apparatus of claim 1, wherein the second plurality of recesses have a cross-section that is one of substantially rectangular and substantially triangular.
 9. The lithographic apparatus of claim 1, wherein the second spectral purity filter is a grating filter.
 10. A device manufacturing method comprising: patterning a beam of radiation and projecting the beam of radiation onto a substrate; using a first spectral purity filter to reduce the intensity of radiation in the beam of radiation in at least one first undesirable range of radiation wavelengths; and using a second spectral purity filter to reduce the intensity of radiation in the beam of radiation in at least one second undesirable range of radiation wavelengths that is different than the at least one first undesirable range of radiation wavelengths; wherein the first spectral purity filter and second spectral purity filter are provided with different radiation filtering structures from each other, wherein the first spectral purity filter is a grating spectral purity filter comprising: a multilayer stack of alternating layers, configured to reflect radiation of a first wavelength in a first direction relative to the multilayer stack; and a first plurality of recesses in a top side of the multilayer stack, the first plurality of recesses configured to form a grating arranged such that radiation of a second wavelength is reflected in a second direction relative to the multilayer stack that is different from the first direction; and wherein the second spectral purity filter comprises a second plurality of recesses, smaller than the first plurality of recesses, formed on the top side of the multilayer stack between the recesses of the first plurality of recesses and in a lower surface of the recesses of the first plurality of recesses, wherein the second plurality of recesses are configured to form at least one second grating arranged such that radiation of a third wavelength is reflected in a third direction relative to the multilayer stack that is different from the first direction.
 11. A spectral purity filter, comprising: a multilayer stack of alternating layers, configured to reflect radiation of a first wavelength in a first direction relative to the multilayer stack; and a first plurality of recesses in a top side of the multilayer stack, the first plurality of recesses configured to form a first grating arranged such that radiation of a second wavelength is reflected in a second direction relative to the multilayer stack that is different from the first direction; characterized by a second plurality of recesses, smaller than the first plurality of recesses, formed on the top side of the multilayer stack between the recesses of the first plurality of recesses and in a lower surface of the recesses of the first plurality of recesses, the second plurality of recesses configured to form at least one second grating arranged such that radiation of a third wavelength is reflected in a third direction relative to the multilayer stack that is different from the first direction.
 12. The spectral purity filter of claim 11, wherein the first plurality of recesses have a depth of between approximately 1.5 μm to 3 μm.
 13. The spectral purity filter of claim 12, wherein the depth is approximately 2.65 μm.
 14. The spectral purity filter of claim 11, wherein the second plurality of recesses have a depth of between approximately 25 nm to 75 nm.
 15. The spectral purity filter of claim 14, wherein the depth is approximately 50 nm.
 16. The spectral purity filter of claim 11, wherein the second plurality of recesses have a cross-section that is one of substantially rectangular and substantially triangular.
 17. The spectral purity filter of claim 11, wherein the second and third directions are substantially the same. 